A spectrum analyser is a measuring instrument, which quantifies the energy, or the electromagnetic power contained in each frequency of the electromagnetic spectrum of a signal to be analysed. Such analysers typically operate in a frequency range from a few hundreds hertz or less to a few gigahertz. This instrument is used to design radio frequency and microwave systems and is particularly useful for measuring the electromagnetic perturbations, which disturb wireless transmissions. It is also used for verifying and tuning communication systems installed on high point like specialised towers, high buildings and the like. For these reasons and because wireless transmission is a fast growing business, weight and volume become more and more important for security and working conditions of users.
To determine the spectrum e.g. the power at each frequency, between a minimum frequency fmin and a maximum frequency fmax, it is necessary to scan the operating spectrum with a band-pass filter and to measure the power at the output of the filter. The bandwidth of the filter has to be variable to be able to separate close signals or to scan the entire bandwidth rapidly. As no technology is available to make this filtering directly in the entire frequency range, the signal to be analysed is usually converted to a given frequency, by heterodyning, and is filtered accordingly. The minimum value and the maximum value of the filter's bandwidth depends on the general performance of the instrument. For a medium class instrument, the filter is variable from 300 Hz to 3 MHz with step ratio 1, 3, 10, etc.
The frequency converter is basically a mixer. A mixer is a component well known in the art and which is a multiplier of signals, these signals can be decomposed in sum of sinusoidal signals as Fourier demonstrates. A mixing process is then a multiplication of sinusoidal signals. For the simplicity of the discussion, the mixer is considered perfect, i.e. only the first order of the multiplication is considered. We will use the simple well known equationsin(A)˜sin(B)=0.5˜[cos(A−B)−cos(A+B)]In which sin A is a sinusoidal signal at the input of the mixer and B a sinusoidal signal supplied by a local oscillator, hereafter abbreviated LO.
FIG. 1 shows partly of a prior art device, used in the front end of commercially available spectrum analyser like the ones sold by the applicant. The problems to be solved according to the invention is discussed in reference to this figure, using a simple numerical example. The device of FIG. 1 comprises a mixer 2, that mixes the signal to be analysed and the output of a local oscillator 4 at a frequency fol. The output of the mixer is applied to a band-pass filter 6, filtering around a frequency fi. A second mixer 16 and a second local oscillator 15 convert this frequency fi—called intermediate frequency IF—to the final frequency for which the technology exists for an easy and efficient analysis. A filter 17 eliminates the undesired frequency at the output of the second mixer. A direct conversion needs a very high selectivity of filter 6 which in most cases is not suitable for the type of instruments described here.
The operation of the device of FIG. 1 is the following. Local oscillator 4 and mixer 2 convert signals received by the converting device into a signal at the intermediate frequency fi. At this frequency, bandpass filter 6 eliminates the undesired frequencies called spurious frequencies created by the mixing in the mixer. The mixer creates a signal at the frequency fi when the input signal frequency is lower and higher than the frequency of the first local oscillator, specifically:|fol−fin|=fi  (1)where fin is a frequency ray in the signal input to the mixer.
For determining whether there exists a ray at a given frequency fin in the signal applied to the device of FIG. 1, the frequency of the local oscillator is controlled, so thatfol−fin=fi  (2)
Thus, when the local oscillator is set at a frequency fol=fi+fin, a signal at the output of filter 6 is representative of a ray at fin in the signal applied to the mixer.
A frequency fin′, called image frequency, may also create a ray at a frequency fi at the output of mixer 2 and filter 6, when the frequency of the oscillator is selected according to (2). Indeed, assuming thatfol=fin+fi  (2′)then a ray at a frequencyfin′=fol+fi=fin+2·fi  (3)in the signal applied to the mixer, will also generate a ray at fi at the output of the mixer. Indeed, the mixer provides signals atfin′−fol=fi
Thus, for a given value of the frequency of the oscillator, a ray at the output of the mixer may be representative of the searched frequency fin, but may also indicate that there exists an image frequency. The image frequency is offset of 2·fi with respect to the searched frequency.
If the oscillator frequency is swept for providing a spectrum of the input signals, then, for a ray at a given frequency fin in the input signal, a ray appears atfol=fin+fi andfol′=fin−fi 
This provides two rays at the output of the mixer per signal at the input.
For instance, assuming the intermediate frequency is 400 MHz. Assuming there is an actual ray at fin=10 GHz in the input signal. For searching rays around 10 GHz in the input signal, the oscillator is set at a frequency fol=10.4 GHz. A ray then appears at the output of the mixer: this is the expected ray. However, a ray in the spectrum at fin′=10.8 GHz would also cause a signal to appear at a frequency of 400 MHz at the output of the mixer, when the oscillator is set at fol=10.4 GHz.
In a sweeping mode, assume there exists a ray at fin=10 GHz in the input signal. A signal at the output of the mixer appears when fol is 10.4 GHz. However, a signal at the output of the mixer also appears when the oscillator is set at a frequency fol=9.6 GHz.
In both case, there is a problem of image ray.
One solution to this known problem is to filter the signal applied to the converter, using a settable filter 1 known as a pre-filter. In the example given above, when the frequency of the oscillator is set at 10.4 GHz, the input signal could be filtered around 10 GHz, for avoiding the image frequency at 10.8 GHz. This solution is rather costly and difficult to implement over a broad range of frequencies, e.g. from 3 to 50 GHz and from our knowledge no commercial product exists up to now for a wider range. This pre-filter 1 is also heavy, rather big and need a high power consumption which is a big penalty for a mobile system working on batteries. Note that when using a pre-filter 1, as seen above, the first IF frequency is generally around 400 MHz because of the limited quality factor of pre-filter 1 which has to filter the image rays. This explains the need of a second mixer and a second oscillator.
Another solution is currently used in the spectrum analysers sold by the applicant under reference R3273 for the very high frequencies above 30 GHz. Two successive sweep results are superimposed for analysing a given spectrum. In the first sweep, one assumes thatfol−fin=fi  (2)and one displays on the screen, where x coordinates is frequency, the rays at frequenciesfol−fi=fin  (4)Any time there is a signal at the output of the mixer when the oscillator is set at a frequency fol, a ray proportional to its power level is displayed on the screen. Assuming there is a ray at fin in the signal input to the mixer, there is a signal at the output of the mixer whenfol−fin=fi  (2)and whenfin−fol=fi  (5)
In case of (2), one displays a ray on the screen at fin; in the case of (5), one displays on the screen a ray at fin−2·fi, which is an image ray not corresponding to an actual ray in the signal input to the mixer.
In the second sweep, one assumes thatfin−fol=fi  (5)and one displays on the screen a ray at a frequencyfol+fi=fin  (6)any time there is a signal at the output of the mixer when the oscillator is set at a frequency fol. Assuming there is a ray at fin in the signal input to the mixer, there is a signal at the output of the mixer whenfol−fin=fi  (2)and whenfin−fol=fi  (5)
In case of (5), one displays a ray on the screen at fin; in the case of (2), one displays on the screen a ray at fin+2·fi, which is an image ray not corresponding to an actual ray in the signal input to the mixer.
For a given spectral ray at fin, when the two sweeps are successively displayed on the same screen, there appears on the screen three rays:                one ray at a frequency fin, which is present in both sweeps;        one ray at a frequency fin′=fin−2·fi, which appears in one of the sweeps;        one ray at a frequency fin″=fin+2·fi, which appears in one of the sweeps.        
The first ray is stable on the screen; the other two rays flicker, since they only appear in one of the two sweeps. This makes it possible to distinguish the image rays, and locate the frequency rays actually existing in the input signal.
Assuming again the first intermediate frequency is 400 MHz. Assuming there exists a ray at fin=10 GHz in the input signal. In the first sweep, a ray appears at the output of the mixer when the oscillator is set at a frequency fol=10.4 GHz, which corresponds to the expected ray at 10 GHz. However, there is also a signal at the output of the mixer when fol=9.6 GHz, that is for the image ray in the spectrum at fin′=9.2 GHz. In the first sweep, rays at 9.2 and 10.0 GHz are displayed.
In the second sweep, a ray appears at the output of the mixer when the oscillator is set at a frequency fol=9.6 GHz, this corresponding to the expected ray at fin=10 GHz; another ray is present at the output of the mixer when fol=10.4 GHz, that is for the image ray in the spectrum at fin″=10.8 GHz. In the second sweep, rays at 10.0 and 10.8 GHz are displayed.
When the two sweeps are displayed at the refresh speed of the instrument, rays at 9.2 and 10.8 GHz are flickering, while the ray at 10.0 GHz is stable. This makes it possible for the user to distinguish the actual rays, and to realise that the other two rays are artefacts.
The invention is based on the following new problem: under specific circumstances, the prior art operation under two sweeps may not prove satisfactory. For instance, assume two rays are present in the input signals. For certain spacing between the rays, there may appear stable rays which actually do not represent any actual ray in the input signal, but simply represent spurious responses that are identical for both actual rays.
Specifically, assume the input signal has two rays at fin1 and fin2, with fin1<fin2. One sweep generates rays at fin1, fin2, fin′1=fin1−2·fi and fin′2=fin2−2·fi; the second sweep generates rays at fin1, fin2, fin″1=fin1+2·fi and fin″2=fin2+2·fi. Assuming for instance:fin′2=fin″1  (7),that is:fin1+2·fi=fin2−2·fi fin2=fin1+4·fi 
In this case, there appears on the screen three stable rays, and two flickering rays.
With the same example of 400 MHz, assume there are two rays at fin1=10 GHz and fin2=11.6 GHz in the input signal. In the first sweep, a ray appears at the output of the mixer when the oscillator is set at a frequency fol=10.4 GHz, this corresponding to fin1. However, there is also a signal at the output of the mixer when fol=9.6 GHz, that is for the image ray in the spectrum at fin′1=9.2 GHz. Similarly, for the second ray at fin2, a ray appears at the output of the mixer when the oscillator is set at a frequency fol=12.0 GHz, this corresponding to fin2; there is also a signal at the output of the mixer when fol=11.2 GHz, that is for the image ray in the spectrum at fin′2=10.8 GHz.
In the second sweep, a ray appears at the output of the mixer when the oscillator is set at a frequency fol=9.6 GHz, this corresponding to fin1. However, there is also a signal at the output of the mixer when fol=10.4 GHz, that is for the image ray in the spectrum at fin″1=10.8 GHz. Similarly, for the second ray at fin2, a ray appears at the output of the mixer when the oscillator is set at a frequency fol=11.2 GHz, this corresponding to fin2; there is also a signal at the output of the mixer when fol=12.0 GHz, that is for the image ray in the spectrum at fin″2=12.4 GHz.
Thus, stable rays at 10, 10.8 and 11.6 GHz appear on the screen; flickering rays at 9.2 and 12.4 GHz also appear on the screen. However, the ray at 10.8 GHz is also an image ray: it is an image ray of the second actual rays in the second sweep, and an image of the first actual ray in the first sweep.
This new problem is of course not limited to two actual rays, but is also present for more than two rays, wherever a pair of rays is spaced by a frequency corresponding to 4·fi.